In addition, halogens act as oxidizing agents / they exhibit the property to oxidize metals. The principle oxidation states of chlorine, bromine and iodine are -1, +1, +3, +5, +7. Fluorine exhibits the oxidation state -1 (fluoride F- ion). All halogens possess the oxidation state 0 in their diatomic elemental forms. Electonegativity, ionization energy, electron affinity and reactivity decrease down the group. That is why halogens form ionic compounds with alkali metals and earth-alkaline metals. This pronounced tendency of the halogens to accept electron renders them strong oxidizers. These electronic configurations are extremely stable. Potentially, a halogen atom could hold one more electron (in p orbital), which would give the resulting halide ion the same configuration as that of the noble gas next to it in the periodic table. The general electronic configuration of outermost shell of halogens is ns2np5. These seven outermost electrons are in two different kinds of orbital, designated s (with 2 electrons) and p (with 5 electrons). The halogen atoms carry seven valence electrons in their outermost electron shell. The chemical behavior of halogen elements depend on the electronic configuration in the valence shell. Small amounts of astatine (As) exist in the nature as a result of the decay of uranium and thorium, although the total amount of astatine in the Earth's crust at any particular time is less than 30 grams. The oxidation states are -1, +1, +3, +5, +7, and the number of stable isotopes is 1. The estimated crustal abundance is 4.5x10-1 mg/kg and the estimated oceanic abundance is 6x10-2 mg/kg. Trace amount of iodine are required for normal function of human body. Iodine (I) is chiefly obtained from nature deposits of sodium iodate (NaIO3) and sodium periodate (NaIO4). The oxidation states are -1, +1, +3, +4, +5, and the number of stable isotopes is 2. The estimated crustal abundance is 2.4mg/kg and the estimated oceanic abundance is 6.73x101 mg/kg. Elemental bromine is hazardous substance. Bromine (Br) is the only non-metallic element that is a liquid at standard temperature and pressure. The most familiar and abundant chlorine compound is sodium chloride (NaCl, table salt) which may be found in sea water and salt mines. The oxidation states are -1, +1, +3, +5, +7, and the number of stable isotopes is 2. The estimated crustal abundance is 1.45x102mg/kg and the estimated oceanic abundance is 1.94x104 mg/kg. Since it combines directly with nearly every element, chlorine (Cl) is never found free in nature. Fluorine is the most electronegative element in the periodic table. The oxidation state is -1 and the number of stable isotopes is 1. The estimated crustal abundance is 5.85x102mg/kg and the estimated oceanic abundance is 1.3mg/kg. For this reason, fluorine doesn't occur free in nature and was extremely difficult for scientist to isolate. Fluorine (F) is the most reactive of all elements and no chemical substances is capable of freeing fluorine from any of its compounds. The halogens may not be found free, in their elemental states, therefore they are always combined in their compounds in nature.
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